1. Field of the Invention
The present invention relates to a micro-electromechanical structure with self-compensation of the thermal drifts caused by thermomechanical stress.
2. Description of the Related Art
The following description will make reference, to an inertial sensor, in particular to a linear accelerometer, without losing generality. As is known, micromachining techniques enable micro-electromechanical structures (MEMS) to be obtained within layers generally of semiconductor material, which have been deposited (for example, a polycrystalline silicon layer) or grown (for example, an epitaxial layer) on top of sacrificial layers, which are removed via etching.
In particular, inertial sensors obtained using micromachining techniques comprise mobile regions (rotor regions) suspended with respect to a substrate, and fixed regions (stator regions) anchored and fixed to the substrate and in particular to the package of the sensor. The rotor regions are connected to the substrate via elastic biasing elements (springs) and consequently are mobile with respect to the stator regions along one or more axes, which define the detection axes of the sensor.
The various regions that make up the micro-electromechanical structure can have different coefficients of thermal expansion, especially when they are subjected to different doping levels. Furthermore, the material of the package of the micro-electromechanical structure has a different coefficient of thermal expansion with respect to the material of the structure (generally mono- or polycrystalline silicon). Consequently, the silicon die of the microstructure at the end of the machining operations may be subjected to residual thermomechanical stress (a phenomenon known as “die warpage”), and in particular the mobile masses may undergo small relative displacements with respect to the fixed regions.
The presence of residual stress leads to considerable problems for the proper operation of micro-electromechanical structures, in particular of micro-electromechanical sensors. For example, in the case of micro-electromechanical structures comprising a mobile mass equipped with a plurality of anchoring points, the thermomechanical stress acting in a different and non-uniform way on the various anchoring points tends to create tensile and compressive stresses and to modify the positions of the various parts of the structure with respect to one another. This leads to alterations in the performance of the sensors, in particular measurement errors and drifts, which are moreover variable according to the production lot and at times also among sensors belonging to a same production lot.
In order to compensate for the aforementioned measurement drifts, a wide range of solutions has been proposed. In particular, generally, these solutions compensate electronically the thermal drifts of the measurement supplied by the micro-electromechanical sensor by adding appropriate electronic components in the reading interface associated to the sensor.
For example, one solution envisages the use of a temperature sensor in the reading electronics associated to the micro-electromechanical sensor. Once the temperature is known, the drifts of the system are compensated electronically using compensation curves previously obtained from appropriate calibration procedures. This solution proves particularly burdensome in so far as it calls for costly and delicate measurement procedures to obtain compensation curves that accurately map the thermal drifts of the sensors, as well as appropriate compensation operations.
WO03/106927 proposes the insertion of a diode in the reading electronics. By exploiting the known variation in temperature of the voltage drop of the diode, the output of the sensor is compensated by combining it with a value proportional to the voltage drop of the diode. A solution of this type is, however, valid only when the voltage drop of the diode effectively has a temperature variation comparable with the output signal of the micro-electromechanical sensor; however this situation does not occur in general, because of the structure and doping differences.